1. Field of the Invention
The present invention relates to a III-V compound semiconductor device, and in part icular, to a Gunn diode. More specifically, the present invention relates to a structure for efficiently operating a Gunn diode in a millimeter wave band.
2. Description of the Related Art
A Gunn diode renders electrons in the conduction band in a hot state (i.e., a state in which the electrons have an energy higher than that in a thermal equilibrium state) by using a high electric field, thereby causing the electrons to transfer from the .GAMMA.-valley in the conduction band to the L- or X-valley. The Gunn diode thus generates a compression wave of the electrons, thereby oscillating at a high frequency. Therefore, the Gunn diode is used as a high-frequency oscillator.
FIG. 3 is a schematic cross sectional view of a structure of a conventional general GaAs Gunn diode 30. FIG. 4 is a schematic thermal equilibrium energy band diagram of the structure of FIG. 3, showing an energy level E.sub.v at the upper end of the valence band, and respective energy levels E.sub.c.GAMMA. and E.sub.CL at the .GAMMA.- and L-valleys of the conduction band. It should be noted that a basically similar structure and energy band structure can be obtained even when InP is used for the Gunn diode in place of GaAs.
The Gunn diode 30 of FIG. 3 has a layered structure including at least a highly doped (e.g., about 5.times.10.sup.17 cm.sup.-3) n.sup.+ GaAs cathode layer 33 (thickness: about 1,000 .ANG.), a relatively lightly doped (e.g., about 2.times.10.sup.16 cm.sup.-3) n-type GaAs active layer 34 (thickness: about 10,000 .ANG.), and a highly doped (e.g., about 5.times.10.sup.18 cm.sup.-3) n.sup.+ GaAs anode layer 35. An Au/Ge/Ni cathode ohmic contact layer 31 is formed on the cathode layer 33 with an n.sup.+ GaAs cap layer 32 (doping level: about 5.times.10.sup.18 cm.sup.-3 ; thickness: about 1,000 .ANG.) interposed therebetween (it should be noted that the cap layer 32 may be omitted). Moreover, an Au/Ge/Ni anode ohmic contact layer 36 is formed under the anode layer 35.
In the structure shown in FIG. 3, when an appropriate bias voltage is applied between the cathode ohmic contact layer 31 and the anode ohmic contact layer 36, a transferred electron effect causes high-energy electrons within the conduction band to transfer from the .GAMMA.-valley to the L- or X-valley. This leads to dynamic space-charge variations within the active layer 34, resulting in current oscillations. Due to the low mobility of the electrons which have transferred to the L-valley, the region where such electrons exist serves as a high-resistance layer, thereby forming an electrical dipole layer. This is generally referred to as a "domain". Oscillations occur due to the domain transfer from the cathode layer 33 to the anode layer 35. The oscillation frequency is determined by the transfer distance of the domain, whereas the oscillation efficiency (i.e., an operation efficiency of the Gunn diode) is determined by a dynamic transfer rate of the electrons from the .GAMMA.-valley to the L- or X-valley. The presence of the "dead zone" causes reduction in the oscillation efficiency (i.e., the Gunn diode operation frequency) due to its parasitic resistive effect.
In the structure of the conventional Gunn diode 30 shown in FIG. 3, electrons having entered the active layer 34 from the cathode layer 33 have a low average energy, and a region of the active layer 34 in the vicinity of the interface with the cathode layer 33 has a low electric-field intensity. Accordingly, the inter-valley transfer rate is low in the region of the active layer 34 in the vicinity of the interface with the cathode layer 33. Such a region is referred to as a "dead zone", since it does not contribute to an active operation of the Gunn diode 30. Such a dead zone is "parasitic" for the entire structure of the Gunn diode 30, since it disadvantageously affects the operation of the Gunn diode 30, in such a manner as to increase electrical resistance components, reduce or prevent oscillations, and the like.
The oscillation frequency of the Gunn diode 30 is determined by the electron transit-time within the active layer 34, and hence, is directly dependent on the length of the active layer 34. Accordingly, the presence of the dead-zone within the active layer 34 leads to a reduction in the maximum oscillation frequency which can be achieved by the simple conventional structure of the Gunn diode 30, and hence, to a reduction in an operating frequency of the Gunn diode 30. As a result, an operation efficiency (i.e., oscillation efficiency) of the Gunn diode 30 is reduced especially in a high-frequency band.
The presence of the dead zone must be sufficiently considered especially in a Gunn diode which oscillates in a high-frequency band (and hence, must have a short active layer), since such a high-frequency-oscillating Gunn diode has a large relative-ratio of a length of the dead zone to the total length of the active layer.
In order to overcome the above-mentioned problems relating to the dead zone, a "hot-electron injector structure" is sometimes used.
FIG. 5A schematically shows one example of a layered structure of a Gunn diode 50 having such a hot-electron injector structure. FIG. 5B is a schematic thermal equilibrium energy band diagram of the structure of FIG. 5A, showing an energy level E.sub.v at the upper end of the valence band, and an energy level E.sub.c at the lower end of the conduction band.
The Gunn diode 50 has a layered structure including at least a highly doped n.sup.+ GaAs cathode layer 51, a graded AlGaAs wide-bandgap layer 52 in which an Al mole fraction varies in a graded manner, an AlGaAs wide-bandgap layer 53 having a fixed composition, a relatively lightly doped n-type GaAs active layer 54, and a highly doped n.sup.+ GaAs anode layer 55. In such a structure, electrons are injected with a high energy from the cathode layer 51 toward the active layer 54 through the AlGaAs wide-bandgap layers 52 and 53 which are interposed between the cathode layer 51 and the active layer 54. As a result, inter-valley transfer occurs more frequently and quickly in the GaAs active layer 54.
In the structure of the Gunn diode 50 shown in FIG. 5A, the AlGaAs wide-bandgap layers 52 and 53 are considered to be a part of the cathode layer 51.
Furthermore, FIG. 6A schematically shows a layered structure of another Gunn diode 60 having a hot-electron injector structure, as disclosed in Japanese Laid-Open Publication No. 58-122791. FIG. 6B is a schematic thermal equilibrium energy band diagram of the structure of FIG. 6A, showing an energy level E.sub.v at the upper end of the valence band, and an energy level E.sub.c at the lower end of the conduction band.
The Gunn diode 60 has a layered structure including at least a highly doped n.sup.+ GaAs cathode layer 61, an AlGaAs wide-bandgap layer 62 having a fixed composition, a relatively lightly doped n-type GaAs active layer 63, and a highly doped n.sup.+ GaAs anode layer 64. In such a structure, electrons are injected with a high energy from the cathode layer 61 into the active layer 63, while passing through the AlGaAs wide-bandgap layer 62 interposed between the cathode layer 61 and the active layer 63 due to the tunneling effect. As a result, inter-valley transfer occurs more frequently and quickly in the GaAs active layer 63.
In the structure of the Gunn diode 60 shown in FIG. 6A as well, the AlGaAs wide-bandgap layer 62 is considered to be a part of the cathode layer 61.
Each of the conventional Gunn diodes 50 and 60 having the hot-electron injector structure is intended to increase inter-valley transfer of the electrons in a region of the GaAs active layer 54 or 63 in the vicinity of the cathode layer 51 or 61, by increasing an energy of the electrons injected into the GaAs active layer 54 or 63. As described above, each of the AlGaAs wide-bandgap layers 52, 53 and 62 of the respective Gunn diodes 50 and 60 is considered to be a part of the cathode layer 51 or 61, rather than a part of the active layer 54 or 63. In fact, the inter-valley transfer of the electrons does not occur within the AlGaAs wide-bandgap layers 52 and 53 as well as within the AlGaAs wide-bandgap layer 62.
Accordingly, the conventional Gunn diodes 50 and 60 having the respective hot-electron injector structures as shown in FIGS. 5A and 6A do not sufficiently overcome the problem relating to the presence of the dead zone within the active layer, i.e., a limited operation efficiency (oscillation efficiency) at the maximum oscillation frequency as well as in a high-frequency band due to the presence of the dead zone.